The present invention relates to scanning electron microscopes (SEM) and, more particularly, to a method and apparatus for performing SE spectroscopy to determine the material composition of an observed sample.
Scanning electron microscopes (SEM) are very useful for imaging very small elements on a sub-micron scale, with a resolution on the order of nanometers. Therefore, various SEM systems are used in the semiconductor industry for engineering and metrology. Recently, much attention has also been given to the use of SEM in investigation of defects on semiconductor circuits. Since the size of defects of interest (i.e., killer defects) continues to shrink with the shrinking of the design rules, there""s a continuous need for improvement in the images obtained by such SEM.
As is well known to those skilled in the art, SEM images are obtained by directing a primary electron beam (PE) onto the sample, and using detectors to collect electrons returned from the sample. Some of these electrons are elastically reflected, while other like secondary electrons (SE) and back scattered electrons (BSE) are emitted from the sample as a result of inelastic collisions. The ratio of the number of emitted electrons to the primary electrons is referred to in the art as the xe2x80x9celectron yield.xe2x80x9d This quantity is characteristic to the material making the sample.
For better understanding of the discussion that follows, the reader""s attention is directed to FIG. 1, depicting the energy spectrum of electrons emitted from a sample upon the impingement of a primary electron beam (the plot is adopted from Image Formation in Low-Voltage Scanning Electron Microscopy, L. Reimer, SPIE, Vaaol. TT12). It is conventionally accepted that emitted electrons having energy up to 50 eV are SE, while emitted electrons having energy above 50 eV are BSE. Notably, the energy distribution of the SE is hardly influenced by the primary beam.
As is known in the art, and as can be understood from the above noted works, SEM images are generally created from SE or BSE depending on the purpose of the study. That is, when the study requires the ability to distinguish between different materials in the sample, BSE detectors are used since their emission is highly dependent on the sample""s material composition. On the other hand, when it is important to understand the topography of the sample, SE detectors are used since their emission is highly dependent on the sample""s topography.
An issue of interest for the semiconductor industry is obtaining a clear contrast of materials. Specifically, theoretical and experimental works showed the capabilities of using BSE for distinguishing between heavy elements, but BSE lacked this capability for the light elements. Therefore, the prevalent methods of obtaining material composition information is by x-ray or auger spectroscopy. However, both methods are too slow for in-line fabrication monitoring. Additionally, x-ray generally requires high PE potential, which may destroy or degrade the sample.
Thus, a better method is needed for distinguishing between elements, especially when similar, light, elements are present within the investigated sample. Such distinction is especially beneficial for identifying the composition of particles and other defect on semiconductor wafers, so that the origin of the defect may be identified.
Various works have tried to correlate the spectrum of secondary electrons to the material composition of the sample. It has been observed that, generally, SE spectrums obtained from insulators have a narrower peak that is situated at lower energies, as compared to SE spectrum of metals. This is depicted in FIG. 2, wherein N(E), the peak intensity, reflects the SE yield. However, to date no system is available to assist the semiconductor fabrication industry in analyzing and monitoring material composition on semiconductor wafers, especially of particles and other defects.
Much of the work that has been done aims at obtaining a complete spectroscopy to completely identify the element comprising the investigated sample. Such work is suitable for engineering analysis and other xe2x80x9cin-depthxe2x80x9d studies, where accuracy and completeness outweigh analysis speed. However, under certain conditions, such as during in-line monitoring of production line, analysis speed if of high importance and may force compromise in accuracy and completeness. Thus, there is a need for a new approach that provides fast analysis results for particular applications.
The present invention provides a solution to the above noted issues by taking a new approach to material characterization. Specifically, the approach of the present invention is especially beneficial when high analysis speed is required, while accuracy and completeness may be targeted to particular level, as acceptable for the particular application.
The present invention provides a system and a method for fast characterization of sample""s material composition, which is especially beneficial for semiconductor fabrication. The present invention characterizes material composition by analyzing secondary electrons emission from the sample. Therefore, the invention is advantageous in that it does not require high PE voltage.
According to one feature of the present invention, electron detector is used to collect secondary electrons emanating from the sample. The detector is controlled to collect a specific narrow band of secondary electrons, and the band is controlled to allow for collection of SE at different energies.
Two modes are disclosed: spot mode and secondary electron spectroscopy material imaging (SESMI). In the spot mode, a spectrum of SE is obtained from a single spot on the sample, and its characteristics are investigated to obtain information of the material composition of the spot. In the SESMI mode, an SEM image of an area on the sample is obtained. The SE spectrum at each pixel is investigated and correlated to a particular spectrum group. The image is then coded according to the SE spectrum grouping. The coding is preferably a color coding.
Various systems for obtaining the SE spectroscopy are provided. Specifically, according to various embodiments contemplated, a retarding potential can be applied to the sample (e.g., the wafer), to the electron column end piece (such as the cap or the lens"" pole pieces), or the detector. Alternatively, a deflection potential can be applied, for example by using a Wein filter. Another possibility is to use a commercial spectrometer, for example a cylindrical mirror analyzer, to record the spectrum of the emitted secondary electrons from the sample.